Section 3.3.1 "Attribute Types" states
XML attribute types are of three kinds: a string type, a set of
tokenized types, and enumerated types. The string type may take any literal
string as a value; . . .
However, in view of the Well-Formedness Constraint
Attribute values can not contain direct or indirect entity
references to external entities.
on [41] "Attribute", it would seem that "any literal string" above should be
changed to "any literal string not containing direct or indirect references
to external entities".